Harmonic generator



Dec. v25, 1945. w. E, BRADLEY HARMONIC GENERATOR Filed July 21 1945 am LWL/5507770 bwk/*10 N /6 OUTPUT l1 through a grid condenser I9 and movable oontact 20. A grid-leak resistance 2| is connected between the control grid and ground.

Screen grid I3 here functions as the plate of the oscillator and is connected with the lower terminals of coil I1 and tuning condenser I8 through a blocking condenser 22.

The positive end of the B potential source is connected to the screen grid I3 through asuitable resistance 23 and to plate II through a resistance 24.

It will be observed that the feed-back path includes condenser 22 and that portion of coil I1 which lies between points 25 and 26. Itwas found that harmonics, as well as fundamental frequency currents, were injected into the tank circuit I6 via this feed-back path and that corresponding harmonic potentials were thus impressed upon the control grid to such extent as to cause the two aforementioned diculties.

The filter, by means of which the feed-back of harmonics is successfully suppressed or attenuated, comprises a series resistance 21 and shunt condenser-'28. For the particular range of fundamental frequencies under consideration, namely 125-250 k. c., the magnitude of resistance 21 may be 5000 ohms while the capacity of conconder 28 may be 250 mmf. At 125 k. c. the impedance of condenser 28 is approximately 5000 ohms. Hence, at the low end of the fundamental frequency scale the impedance of the shunt path comprising condenser 28 is substantially equal to that of the series path including the resistance 21, disregarding the impedance of that portion of coil I1 which is included in series with the resistance 21, and disregarding the skin effect of resistance 21. At the second harmonc and all higher `harmonics the impedance of condenser 28 is much less than at the fundamental frequency, while that of' resistance 21. remains constant except for skin elfect which increases the apparent resistance at the harmonic frequencies, Also the impedance of the series path is increased at the harmonic frequencies due to increased inductive reactance. Manifestly, the filter operatesselectively to permit feed-back of fundamental frequencies without excessive attenuation while eflectively suppressing feedback of all the harmonies.

It will be observed that the I. R. drop across resistance 21 is impressed upon control grid I4 and biases the same negatively. It` is important that the resistance value besuch that the grid bias will be consistent with operation of the tube as a harmonic generator, unless some additional provision is made to correct the grid bias. The R-C values of the filter are not particularly critical. Of course, it is essential that these values be intelligently selected with a view to obtaining substantial suppression of the harmonics without at the same time over-attenuating the necessary feed-back of fundamental frequencies, and it is advisable that the value of resistance 21 be not so low that condenser 28 will in effect shunt a portion of coil I1 and thereby influence the tuning of the tank circuit.

Through the use of a pentode tube, electronic coupling is obtained between the oscillator anode grid I3 and the output anode I0, with the result that the generator operating frequency is not iniiuenced by the load connected to the harmonicv output terminals.

It will be self-evident that the filter which here consists of a single section network could comprise a plurality of sections, thus effecting a sharper cut-off of the harmonics, but I have not yet found it necessary to resort to a multi-section filter. It is possible that for operating on some other band or in employing different tubes a multi-section filter would be desirable. The design and proportioning of such multi-Section filters is a subject so familiar to those skilled in the art and so widely known that it requires no specific treatment here.

The circuit of Fig. 3 was originally employed without the filter comprising resistor 21 and condenser 28 and it was found that the calibration curve instead of being uniformly linear as per Fig. 1 always contained a number of irregularities or aberrations somewhat as depicted in Fig. 2. The reason f or this was discovered to be not merely the fact that harmonics were fed back into the tank circuit, but the further fact that inherent capacities across portions of coil I1-which capacities could not be eliminated-gave rise to a number of parasitic tank circuits which were resonant to various harmonic frequencies; and whenever in the course of tuning the generator a harmonic of a frequency equal to the resonant frequency of` one of said parasitic tank vcircuits was impressed upon coil I1 via the feed-back path an irregularity would occur in the calibration curve. If the generator had been intended to operate at a single fixed frequency instead of being tunable over a considerable range it would have been possible to select a coil I'I which would not be resonant, as a consequence of its inherent distributed capacity, to any of the harmonics of that single frequency. But because of the fact that the tank circuit had to be tunable over a considerable range it was not practicable to so design coil I1 that it would not be self-resonant to some of the numerous harmonic frequencies produced in the course of tuning the generator from one end cf its band to the other. It will be apparent from the foregoingexplanation, that the present invention is especially advantageous only in connection with an oscillation generator which is tunable; that isto say, one which is designed to operate over a band of frequencies, as distinguished from a fixed frequency oscillation generator.

Fig. 4 illustrates a harmonic generator which is nearly identical with Fig. 3 except that it employs a three-electrode tube 29 instead of a pentode tube. Here the tunable tank circuit is identied by reference numeral 30, while 3| is the grid condenser, 32 the grid-leak resistance, 33 a blocking condenser in the feed-back path, 34 a coupling resistance and 35 a blockingcondenser serving to isolate the D. C. plate current from the load. The R-C filter comprises a series resistance element 36 and a shunt condenser 3'I. The functioning of the filter is the same as in Fig. 3. The circuit of Fig. 4 isinferior to that of Fig. 3 because of the absence of theelectronic coupling in the output, but otherwise it is fully equivalent to Fig. 3 and may successfully be employed where the load is not likely to reflect reactance into the oscillator circuit and thus iniiuence the output frequency.

For the purpose of demonstrating that the invention is not limited to oscillation generators of the Hartley type, I have shown in Fig. 5 an application thereofv to the well-known Colpitts-type oscillator. Here the tunable tank circuit 38 includes an inductance coil 39 and two variable condensers 40, 4 I. Tube 42 is a pentode whereof the n cathode 43 is connected to the midpoint 44 through the nlter resistance 45 and to the neg- `cies and to substantially reject all harmonics of accuses s ative terminal of the B supplythrough a radio l frequency choke coil 46 shunting the ltercondenser 41. The usual grid condenser 48 and gridleak resistance 49 are inserted in the control grid lead. Shield grid 50 functions as the oscillator plate and is connected to the positive terminal of the B supply through a resistance I while the plate 52 is connected to the same terminal through a resistance 53. A blocking condenser 54 is included in the feed-back path. The harmonic frequency output canbe taken oi as indicated or from the terminals of the series-combined resistances 5|, 53.

It will be understood that the several embodiments illustrated and described yare not intended to limit the invention, which may take other forms of physical expression within the scope of theappended claims.

I claim:

1. A variable frequency oscillation generator comprising an amplifying device having input and output terminals, a frequency-determining resonantl tank circuit tunable through a range of oscillation frequencies, and a feed-back path connecting said output terminals with said resonant circuit, said input terminals being coupled to said resonant circuit, said feed-back path including a fixed-constant iilter designed to pass eiciently the aforementioned range of oscillation frequenall said oscillation frequencies.

2. A variable frequency oscillation generator comprising an amplifying device having input and output terminals, a frequency-determining resonant circuit tunable through a range of oscillation frequencies, said input terminals being connected to said resonant circuit, and a feed-back path connecting said output terminals with said resonant circuit, said feed-back path including a fixed-constant filter designed to kpass efliciently the aforementioned range of oscillation frequen-y cies and to substantially reject all harmonics of all said oscillation frequencies, said filter includ'- ing a series resistance and a shunt capacitance, said capacitance being isolated from said resonant circuit by said series resistance.

3. A variable frequency oscillationy generator comprising a vacuum tube amplifier having input and output terminals, a frequency-determining resonant tank circuit tunable through a range of oscillation frequencies, a'portion of said tank circuit being connected across said input terminals, afeed-back path including in series a second portion of said tank circuit and connecting said output terminals with said tank circuit, said feed-back path including a fixed-constant filter comprising series resistance and shunt capacity,

said filter being designed to pass efficiently the oscillation frequencies within said range and to substantially reject all the harmonics of said oscillation frequencies. l y

" 4. A harmonic frequency generator comprising a vacuum tube amplifier having input and output terminals, a frequency-determining tank circuit tunable over a band of fundamental frequencies and having a portion connected across said input terminals, a feed-back path including in series a second portion of said tank circuit and connecting said output terminals with said tank circuit, said feed-back path including a fixedconstant lter for selectively suppressing the `.passage into said tank circuit of all harmonics of 'all said fundamental frequencies within said range Without commensurately suppressing the passage'of fundamental frequency currents into said tank circuit. Y

5. A harmonic frequency generator comprising a vacuum tube amplifier having input and output terminals, a tunable frequency-determining tank circuit including two condensers connected in series across the terminals of an inductance coil,

said input terminals being connected across one of said condensers, said output terminals being connected across the other of said condensersV through a feed-back path, and a fixed-constant filter in said feed-back path, said filter being characterized in that it passes efiicientlyrall of the frequencies to which said tank circuit is tunable and further characterized in that it effectively suppresses all the harmonics of all said fre- 5- quencies.

WILLIAM E. BRADLEY. 

